1. Field of the Invention
This invention relates to thermal-insulating coatings for components exposed to high temperatures, such as the hostile thermal environment of a gas turbine engine. More particularly, this invention is directed to a thermal barrier coating (TBC) of zirconia partially stabilized with less than four weight percent yttria and further alloyed with lanthana, neodymia or tantala to increase the impact resistance of the TBC.
2. Description of the Related Art
Higher operating temperatures for gas turbine engines are continuously sought in order to increase their efficiency. However, as operating temperatures increase, the high temperature durability of the components within the hot gas path of the engine must correspondingly increase. Significant advances in high temperature capabilities have been achieved through the formulation of nickel and cobalt-base superalloys. Nonetheless, when used to form components of the turbine, combustor and augmentor sections of a gas turbine engine, such alloys alone are often susceptible to damage by oxidation and hot corrosion attack, and as a result may not retain adequate mechanical properties. For this reason, these components are often protected by a thermal barrier coating (TBC) system. TBC systems typically include an environmentally-protective bond coat and a thermal-insulating topcoat, typically referred to as the TBC. Bond coat materials widely used in TBC systems include oxidation-resistant overlay coatings such as MCrAIX (where M is iron, cobalt and/or nickel, and X is yttrium or another rare earth or reactive element), and oxidation-resistant diffusion coatings such as diffusion aluminides that contain nickel-aluminum (NiAl) intermetallics.
Zirconia (ZrO2) that is partially or fully stabilized by yttria (Y2O3), magnesia (MgO) or another alkaline-earth metal oxide, ceria (CeO2) or another rare-earth metal oxide, or mixtures of these oxides has been employed as a TBC material. Binary yttria-stabilized zirconia (YSZ) has particularly found wide use as the TBC material on gas turbine engine components because of its low thermal conductivity, high temperature capability including desirable thermal cycle fatigue properties, and relative ease of deposition by plasma spraying, flame spraying and physical vapor deposition (PVD) techniques such as electron beam physical vapor deposition (EBPVD). TBC's employed in the highest temperature regions of gas turbine engines are often deposited by PVD, particularly EBPVD, which yields a strain-tolerant columnar grain structure that is able to expand and contract without causing damaging stresses that lead to spallation. Similar columnar microstructures can be produced using other atomic and molecular vapor processes, such as sputtering (e.g., high and low pressure, standard or collimated plume), ion plasma deposition, and all forms of melting and evaporation deposition processes (e.g., cathodic arc, laser melting, etc.). In contrast, plasma spraying techniques such as air plasma spraying (APS) deposit TBC material in the form of molten splats, resulting in a TBC characterized by a degree of inhomogeneity and porosity.
As is known in the art, zirconia is stabilized with the above-noted oxides to inhibit a tetragonal to monoclinic phase transformation at about 1000° C., which results in a volume expansion that can cause spallation. At room temperature, the more stable tetragonal phase is obtained and the undesirable monoclinic phase is minimized if zirconia is stabilized by at least about six weight percent yttria. An yttria content of seventeen weight percent or more ensures a fully stable cubic phase. Though thermal conductivity of YSZ decreases with increasing yttria content, the conventional practice has been to stabilize zirconia with at least six weight percent, and more typically to only partially stabilize zirconia with six to eight weight percent yttria (6–8% YSZ). Limited exceptions have generally included plasma-sprayed zirconia said to be stabilized by mixtures of yttria, magnesia, calcia or ceria, to which certain oxides may be added at specified levels to obtain a desired effect. For example, according to U.S. Pat. No. 4,774,150 to Amano et al., Bi2O3, TiO2, Tb4O7, Eu2O3 and/or Sm2O3 may be added to certain layers of a TBC formed of zirconia stabilized by yttria, magnesia or calcia, for the purpose of serving as “luminous activators”.
Contrary to the conventional practice of stabilizing zirconia with at least six weight percent yttria, U.S. Pat. No. 5,981,088 to Bruce et al. unexpectedly showed that if a YSZ coating has a columnar grain structure (e.g., deposited by EBPVD), superior spallation resistance can be achieved if zirconia is partially stabilized by less than six weight percent yttria. Significantly, YSZ TBC's in accordance with Bruce et al. contain the monoclinic phase, which was intentionally avoided in the prior art by the six to eight weight percent yttria.
Commonly-assigned U.S. Pat. No. 6,586,115 to Rigney et al. discloses a TBC of zirconia partially stabilized with yttria, preferably not more than three weight percent yttria (3% YSZ), to which one or more additional metal oxides having an ion size difference relative to zirconium ions (Zr4+) are alloyed to reduce the thermal conductivity of the TBC. The additional metal oxides are disclosed to be limited to the alkaline-earth metal oxides magnesia (MgO), calcia (CaO), strontia (SrO) and barium oxide (BaO), the rare-earth metal oxides lanthana (La2O3), ceria (CeO2), neodymia (Nd2O3), gadolinium oxide (Gd2O3) and dysprosia (Dy2O3), as well as such metal oxides as nickel oxide (NiO), ferric oxide (Fe2O3), cobaltous oxide (CoO), and scandium oxide (Sc2O3). Rigney et al. teaches that the required degree of crystallographic defects and/or lattice strain excludes such oxides as hafnia (HfO2), titania (TiO2), tantala (Ta2O5), niobia (Nb2O5), erbia (Er2O3) and ytterbia (Yb2O3), as well as others.
In addition to spallation resistance and low thermal conductivities, TBC's on gas turbine engine components are required to withstand damage from impact by hard particles of varying sizes that are generated upstream in the engine or enter the high velocity gas stream through the air intake of a gas turbine engine. The result of impingement can be erosive wear (generally from smaller particles) or impact spallation from larger particles. In regard to the YSZ TBC of Bruce et al., it has been observed that impact resistance decreases with yttria contents of less than four weight percent. Commonly-assigned U.S. Pat. No. 6,352,788 to Bruce has identified that YSZ containing about one up to less than six weight percent yttria in combination with about one to about ten weight percent of magnesia and/or hafnia exhibits improved impact resistance. It can be appreciated that it would be desirable if other compositions were available for TBC's that exhibit improved impact resistance, particularly as TBC's are employed on components intended for more demanding engine designs.